I suspect there is a typo in this question. Perhaps it should say: How much heavier is person A than person C.

now we got these twp equations
a + b = 175 (1)
b = 150 - c (2)

Subbing (2) in to (1)

a + 150 - c = 175

a - c = 25

a = c + 25 (3)

Subbing (3) in to 1
c + 25 + b = 175

c + b = 150

b = 150 - c

Unfortunately, you have just gone round in a circle. You started off knowing that b = 150-c and then did all this work to deduce it again. This isn't a silly technique to use in a question like this, but you need as many equations as you have variables to start off with, otherwise you will be unable to solve completely.

b = 150 - c

This means B is 150 more than c

So i guess C is equal to 0

This does not mean that b is 150 more than c, as Aryth rightly pointed out. What it means is that b is c less than 150. To get an answer to this question you need to get something of the form b = c +x. You would then be able to write "b is x more than c". Unfortunately there is not enough information given to do this. Judging by your working I think you would be able to solve this if you had a question that worked.